This invention relates to parabolic reflectors which may be used as solar energy concentrators in solar energy collecting apparatus but which also may be used in other applications. The present invention also relates to a method of manufacturing parabolic reflectors.
In solar energy collecting apparatus, parabolic reflectors are commonly used for concentrating solar energy on solar energy absorbers. The reflectors which are used are formed of various materials. For example, the reflectors may comprise metal such as aluminium which is highly polished to define a reflecting surface, or glass mirrors. It is important in solar energy collecting apparatus used in the generation of electrical power that the costs of the apparatus be such that the unit power generating costs are competitive with that of conventional power generation systems. The known reflectors used in solar energy collecting apparatus however are generally relatively expensive to manufacture resulting in increased overall costs of the solar energy collecting apparatus. Further if the reflective surfaces of the reflectors become damaged, the whole reflectors are usually required to be replaced which is also costly. Further costs can arise because of loss of power generating capacity in down time in replacing the reflectors.
The present invention aims to provide a parabolic reflector which is relatively inexpensive, which may be easily constructed and which may be easily and rapidly replaced in the event of damage thereto. The present invention further aims to provide a method of manufacturing a parabolic reflector of the above type. The present invention further aims to provide solar energy concentrating apparatus employing parabolic reflectors of the above type. Other objects and advantages of the invention will become apparent from the following description.
The present invention thus provides in a first aspect, a parabolic reflector assembly comprising:
a base member formed of a magnetic material and defining a parabolic surface,
at least one flexible magnet sheet having an inner side and an outer side, said flexible magnet sheet being magnetically bonded to said base member with said inner side of said magnet sheet being adjacent said parabolic surface such that said magnet sheet adopts the parabolic shape of said parabolic surface, and
a reflective material on said outer side of said magnet sheet and defining a parabolic reflective surface.
The reflective material may be in the form of or be carried by a flexible sheet such as a plastics sheet which may be bonded or adhered to the outer side of the magnet sheet. The adherence between the magnet sheet and sheet of reflective material may be defined by self-adhesive layer on the outer side of the magnet sheet, on one side of the reflective material sheet or both.
The reflective material sheet may be formed as a laminate of sheet plastics material and a reflective film such as a silver film or aluminium film. A particularly suitable material is that manufactured by the 3M company and sold under the trademark Silverlux silver film.
As an alternative, the reflective material may be defined by a layer deposited on or otherwise applied to the outer side of the magnet sheet.
The flexible magnet sheet may be any form of flexible permanent magnet and comprise for example a flexible plastic material such as a thermoplastic material which incorporates a magnetic powder or magnetic granules and which is known under the trademark Tromaflex.
The base member for the formation of a parabolic reflector assembly which is of a parabolic trough form may comprise a panel of magnetic sheet metal formed into a parabolic shape to define a parabolic trough surface to receive the magnet sheet which through magnetic attraction to the metal sheet conforms closely to the parabolic trough surface to form a parabolic trough reflector. Suitably the panel of sheet metal is pressed and held against a parabolic formed to form the base member with the parabolic surface. The panel may be secured to the former for example by welding. The base member however may also comprise a parabolic dish member defining a parabolic dish surface to which a plurality of flexible magnet sheets carrying reflective layers or materials may be magnetically bonded to form a parabolic dish reflector assembly.
A layer of a highly heat conducting material may be interposed between the reflective material and magnet sheet. Suitably, the heat conducting material is aluminium or other high heat conducting metal. The highly heat conducting material is used to dissipate heat generated in the reflector. In parabolic reflectors of the above type, the optical concentrators can have imperfect concentrating surfaces which can result in localised areas of high concentration of solar energy and thus areas of high temperature. This can cause damage to the components of the parabolic reflector, particularly the magnet sheet. The use of a highly heat conducting material interposed between the reflective material and magnet sheet dissipates the heat and conducts it more readily through to the parabolic base member. In addition, the highly heat conducting material adds some stiffness to the reflective material/magnet sheet combination such that a more accurate parabolic surface results when the combination is applied to parabolic base members having imperfect parabolic surfaces, for example a galvanised sheet metal base member which may define an imperfect surface due to the galvanising process causing uneven deposits on the surface.
The present invention in a further aspect provides a method of manufacturing a parabolic reflector assembly, said method comprising the steps of:
providing a base member formed of magnetic material, said base member having a parabolic surface,
providing at least one flexible magnet sheet, said magnet sheet having an inner side and an outer side,
providing a reflective material on the outer side of said magnet sheet; and
applying said flexible magnet sheet to said parabolic surface such that said flexible magnet sheet is magnetically bonded to said base member with said inner side of said magnet sheet being adjacent said parabolic surface whereby to cause said reflective material to define a parabolic reflective surface.
The method may include the further step of providing a highly heat conductive material between the magnet sheet and the reflective material.
The method may additionally include the step of forming the base member from a panel of sheet magnetic metal. The panel of sheet magnetic metal may be formed into a parabolic shape by pressing it against a parabolic former and securing it to the former. The panel may be secured to the former by welding.
The present invention in yet a further aspect provides a solar energy collecting apparatus including a solar energy concentrator for concentrating solar energy on a target wherein the solar energy concentrator comprises a parabolic reflector assembly of the above described type.